As the data areal density in hard disk drive (HDD) writing increases, write heads and media bits are both required to be made in smaller sizes. However, as the write head size shrinks, its writability degrades. To improve writability, new technology is being developed that assists writing to a media bit. Two main approaches currently being investigated are thermally assisted magnetic recording (TAMR) and microwave assisted magnetic recording (MAMR). The latter is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008).
Spin transfer (spin torque) devices in MAMR writers are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer, or drive the magnetization into stable dynamics, if the current density is sufficiently high.
Referring to FIG. 1, a generic MAMR writer based on perpendicular magnetic recording (PMR) is depicted. There is a main pole 1 with a sufficiently large local magnetic field to write the media bit 5 on magnetic medium 4. Magnetic flux 8 in the main pole proceeds through the air bearing surface (ABS) 6-6 and into medium bit layer 4 and soft underlayer (SUL) 7. A portion of the flux 8a returns to the write head where it is collected by write shield 2. For a typical MAMR writer, the magnetic field generated by the main pole 1 itself is not strong enough to flip the magnetization of the medium bit in order to accomplish the write process. However, writing becomes possible when assisted by a spin torque oscillator (STO) 3 positioned between the main pole and write shield 2. The STO and medium bit 5 are enlarged in FIG. 1 side (b) and the former is comprised of a high moment magnetic layer 10, and a second magnetic layer 11 called a spin polarization (SP) layer that preferably has perpendicular magnetic anisotropy (PMA). Between layers 2 and 10, 10 and 11, and 11 and 1, there are nonmagnetic layers 12, 13, 14, respectively, made of a metal to prevent strong magnetic coupling between adjacent magnetic layers.
Assuming a medium bit 5 with a magnetization in the direction of 9 (pointing up) is being written by a flux field 8 pointing down as in FIG. 1 side (a), part of the magnetic flux 8b goes across the gap between main pole 1 and write shield 2, and this weak magnetic field can align the magnetization of SP layer 11 perpendicular to the film surface from left to right. An external current source 18 creates a bias current I across the main pole and write shield. The applied dc results in a current flow in a direction from the write shield through the STO 3 and into main pole 1.
Referring to FIG. 2a, the direct current generated by source 18 is spin polarized by magnetic layer 11, interacts with magnetic layer 10, and produces a spin transfer torque τs 23 on layer 10. Spin transfer torque has a value of aj m×m×mp, where aj is a parameter proportional to the current density j, m is the unit vector 15 in the direction of the instantaneous magnetization for layer 10, and mp is the unit vector 16 in the direction of magnetization in layer 11. Spin transfer torque τs 23 has a representation similar to the damping torque τD 24, and with a specific current direction, τs 23 competes with τD 24, so that the precession angle 50 is from about 0 to 10 degrees. Only when the current density is above a critical value j, will τs 23 be large enough to widely open the precession angle of magnetization 15 in layer 10 such that the oscillation has a large angle 51 usually between 60° and 160° as indicated in FIG. 2b. The large angle oscillatory magnetization of layer 10 generates a radio frequency (rf) usually with a magnitude of several to tens of GHz. This rf field 49 (FIG. 1 side b) interacts with the magnetization 9 of medium bit 5 and makes magnetization 9 oscillate into a precessional state 17 thereby reducing the coercive field of medium bit 5 so that it can be switched by the main pole field 8.
The magnetic layer 10 is referred to as the oscillation layer (OL), and the aforementioned oscillation state is also achieved if main pole field 8 and medium magnetization 9 are in the opposite directions to those shown in FIG. 1. In this case, the direction of the SP magnetization 16 will be reversed, and OL as well as the medium bit will precess in the opposite direction with respect to the illustration in FIG. 1 side b.
In the prior art, seed layer 14 is typically thicker than 3 nm to prevent SP layer to main pole (MP) coupling. Moreover, SP layer 11 is normally >8 nm in thickness since laminates such as (Co/Pt)n, (Co/Ni)n, and (FeCo/Ni)n where n is a lamination number of about 5 to 30 are used to establish strong PMA. Total PMA for the SP layer must overcome the perpendicular demagnetization field so that SP magnetization 16 stays in the perpendicular to film plane direction without an external field. Non-magnetic layer 13 is typically 2 nm thick and the thickness of OL 10 is generally >10 nm. If the OL is too thin, magnetic moment 15 of OL cannot deliver a large enough rf field to assist recording. Non-magnetic layer 12 (often called a capping layer in the bottom STO design in FIG. 1b) is preferably >5 nm thick. Thus, a conventional STO stack representing the write gap is at least 28 nm thick. For a stable STO that can function with good yield, another 5-10 nm may need to be allocated to SP 11, OL 10, and capping layer 12 for a total STO thickness well over 30 nm.
Z. Zeng et al. in “High-Power Coherent Microwave Emission from Magnetic Tunnel Junction Nano-oscillators with Perpendicular Anisotropy”, ACS Nano Vol. 6, No. 7, pp. 6115-6121 (2012), and H. Kubota et al. in “Spin-Torque Oscillator Based on Magnetic Tunnel Junction with a Perpendicularly Polarized Free Layer and in-Plane Magnetized Polarizer” in Applied Physics Express (Jap. Soc. of App. Physics), 6, pp. 103003-1-103003-3 (2013) both present ideas for devices such as an rf signal source and rf field detector with high power narrow band rf emission using a magnetic tunnel junction (MTJ) having a magnetic layer with PMA. A common feature is a MTJ that is comprised of a free magnetic layer with PMA that is not based on a laminated structure such as (Co/Ni)n, but rather PMA is established by an interface between a MgO layer and a thin CoFeB or FeB layer. As a result, the free layer can be more easily driven into large angle oscillation with high power and a narrow frequency band. However, the aforementioned MTJ structure cannot be applied in a STO for MAMR applications mainly because the thin free layer has a low magnetic moment due to its thickness of around 2 nm that is required for the induced anisotropy field to overcome its demagnetization field in the perpendicular to plane direction. Even if the free layer can be driven into an oscillation angle at a similar level to that depicted in FIG. 1, total rf field applied to the magnetic medium is only 20% of that produced by a conventional STO for MAMR because of the low magnetic moment.
Clearly, current STO technology must be improved to enable a smaller write gap (WG) without compromising the rf field strength directed onto the magnetic medium in a MAMR application. PMR writer production already requires a WG to be at 25 nm, significantly less than state of the art STO structures with a WG near 30 nm or larger, in order to achieve a large enough field gradient for good writing performance. Although MAMR improves writing performance, a larger WG than desired reduces the field inside the head between the MP and write shield that is needed for MAMR to work at the required frequency. Therefore, reducing the WG thickness in the STO stack to 25 nm or less becomes a necessity if MAMR is to be widely accepted in the industry.